1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch, such as a torque converter or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode (partially slipping mode) such that an actual amount of slip (slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss due to slipping of the lock-up clutch. The slip control mode is established when the running condition of the vehicle is in a predetermined slip control area which is intermediate between a fully releasing area in which the lock-up clutch should be held in a fully released state, and a fully engaging area in which the lock-up clutch should be held in a fully engaged state. These fully releasing, fully engaging and slip control areas are defined by suitable parameters (e.g., throttle valve opening and vehicle running speed) indicative of the vehicle running condition.
Generally, a lock-up clutch whose slip speed or amount is adjustable is provided with a piston which is operated by a hydraulic pressure source that permits full engagement of the lock-up clutch. Described in detail, the piston is moved depending upon a difference between pressures in two oil chambers, which are formed on the opposite sides of the piston. The amount of slip of the lock-up clutch is controlled by controlling the pressure difference of the two oil cheers to thereby change a thrust force acting on the piston and the resulting friction force of the clutch. Since the hydraulic pressure source that permits the full engagement of the clutch is utilized to control the lock-up clutch in the slip control mode, even a small amount of change in the pressure difference of the two oil chambers will result in a considerable amount of change in the slip amount of the lock-up clutch. That is, the slip amount of the clutch controlled in a feedback fashion tends to be excessively sensitive to a change in a slip control signal generated by a feedback control system. Further, the feedback control of the slip amount of the lock-up clutch suffers from comparatively low control stability, such as a variation in the speed of a vehicle engine due to a low control response upon initiation of the slip control of the lock-up clutch. To avoid engine speed variation, there is proposed a slip control apparatus wherein the slip control output prior to the initiation of the slip control is adjusted so that the pressure difference of the oil chambers is suitable for initiating the slip control without a variation in the engine speed due to the low control response. An example of such a slip control apparatus is disclosed in JP-A-5-99331.
However, the conventional slip control apparatus indicated above suffers from a poor response of the clutch piston to a change in the slip control output during the slip control of the lock-up clutch while the vehicle is in a certain decelerating or accelerating condition. Described more particularly, the pressure difference and the engagement force of the lock-up clutch are small where the target slip speed determined depending upon the vehicle running condition is almost equal to a difference of the input and output speeds of the fluid-filled power transmitting device (=slip speed of the device), which difference would exist or be established when the the lock-up clutch is placed in its fully released state. In this condition, the target slip speed can be attained by a comparatively small change in the slip control output of the slip control apparatus. Where the pressure difference of the lock-up piston is small, therefore, the actual slip speed of the lock-up clutch is not highly responsive to a change in the slip control output, due to a sliding resistance of the clutch piston and an oil leakage from the hydraulic system. Therefore, the feedback control of the pressure difference of the clutch tends to cause a control hunting, leading to deteriorated control stability of the lock-up clutch in certain running condition of the vehicle as indicated above.